MFC r332966: Add deprecation notice for lmc(4)

We intend to remove support before FreeBSD 12 is branched. These are
available only as 32-bit PCI devices. The driver has an ambiguous
license and I have not been successful in contacting the driver's author
in order to address this.

The planned deprecation has been announced on -current and -stable; if
we receive feedback that the driver is still useful and we are able to
resolve the license issue this deprecation notice can be reverted.

Relnotes: Yes
Approved by: re
Sponsored by: The FreeBSD Foundation